Study On Fracture Properties Of Bimetallic Pipe Welded Joints Of X65/Incoloy825

In this paper,the fracture properties of X65/Incoloy825 bimetallic welded joints were studied.Because the X65/Incoloy825 bimetallic coating metal Incoloy825 has thin thickness and the pipeline steel is usually in the service state.The experimental method of the traditional fracture test method is not easy to draw or can not meet the traditional fracture experimental method.Therefore,this paper uses micro-sample test method pre-cracked Small Punch Test(p-SPT)to evaluate the fracture properties of X65/Incoloy825 bimetallic welded joint.The paper first studied the microstructure and properties of the X65/Incoloy825 bimetallic welded joints.The results showed that the typical characteristics of the dissimilar steel joints were the Type-II grain boundary parallel to the fusion line and the Type-I grain boundary between the fused line and the Type-II grain boundary near the fusion line.In addition,by observing the EBSD of the welded joint,the grain size of the X65 side near the fusion line is larger than the grain size far away from the X65,and the weld microstructure shows a columnar crystal growth pattern that is perpendicular to the fusion line.The experimental results of the small punching test of the welded joint show that the crack propagation point of the X65 layer appears at the maximum load from the maximum load of 100% to the maximum load of 95%.In the process of crack propagation,the crack tip first appears along the crack width direction.When the width direction is extended to a certain extent,the crack lengthwise.The crack growth amplitude of specimens with prefabricated crack length in the a=4.3-4.7mm interval can not represent the state of crack initiation,but thespecimen cracks in the a=5.0-5.3mm interval are small and small.Therefore,the experimental numerical value of a=5.0-5.3mm should be adopted in the calculation of the joint J integral by the multi sample method.The other method is to use the CTOD method to calculate the J integral value of the X65 steel base.The calculated J integral value is less influenced by the length of the prefabricated crack,the result is the most concentrated and the discreteness is the smallest,but the disadvantage is that the crack tip opening displacement is very sensitive to the time of the test stop,and the experiment needs to be instantaneous before the crack initiation.A large number of pre experiments are needed before the experiment to determine the crack initiation point.In this paper,the crack initiation point of X65 steel is obtained by a large number of interruption experiments,and the J integral value of X65 steel is calculated by the CTOD method.Finally,the fracture toughness of X65 parent material and weld is tested by standard fracture CT test(compact tensile test).Compared with the calculated numerical value of CT and p-SPT value obtained by the calculation of X65 parent material and weld seam,it is found that the small punching rod experiment has a great fluctuation compared to the CT experiment in calculating the fracture toughness,and uses p-SPT to explore the welding joint.The fracture toughness of the head needs more experiments to explore.